The invention relates to a process for making opaque (or opacification of) a gaseous medium in the optical and infrared (IR) bands of the electromagnetic wave spectrum; it relates also to the application of this process to an electro-optical counter-measure device. Some modern weapons systems are particularly efficient against relatively sensitive objects such as tanks, aircrafts, ships, shelters, etc . . . These weapons systems use projectiles or missiles equipped with electrooptical guidance or homing means which use the energy from the thermal or optical radiation coming from the object aimed at, still called a target in what follows. These radiations of an electromagnetic nature are: either emitted directly by the target itself, and more precisely by its propulsion system, or they result indirectly from the scattering by the target of natural illumination or even of an intentional illumination.
Other weapon systems are electro-optical sighting devices for detecting and locating a target by its natural radiation situated in the IR band (8 to 13 .mu.m), for example, this location of the target permitting the firing of arms of the ballistic or guided type to be triggered off.
To counter the menace formed by electro-optical arms systems, different processes or means capable of deceiving the sighting, measuring and guiding devices of these arms systems are already known and are classified in two counter-measure techniques, one called "active" and the other "passive". According to the active counter-measure technique, electromagnetic radiation situated in the operating band of the weapon system is intentionally emitted, with the purpose particularly of blinding, jamming, or decoying the system. According to the passive counter-measure technique, the end in view is to create a reflecting and/or absorbing medium so as to provide generally in the form of a cloud a protective screen behind which the target to be detected may remain or move; according to the passive technique, the use must also be mentioned, on board the target, of specific means for reducing the radiation level emitted particularly by the propulsion unit or any other heat source.
In the prior technique relating to passive counter-measures, processes have already been proposed for creating "clouds" formed from extremely fine particles. For example, the process has been used of expansion by gas diffusion to create clouds formed from metal microparticles, with specific shapes and sizes having the property of reflecting or scattering incident electromagnetic radiation. Along other lines, attempts have been made to create clouds formed from non-metal particles, with specific shapes and dimensions, these having the property of absorbing the incident electromagnetic radiation.